The following correction was printed in the Guardian's Corrections and clarifications column, Friday August 31 2007

The report below about the murder of Steven Hoskin, referred in error to the Treffry viaduct in the Luxulyan valley. The scene of the murder was the Trenance viaduct in St Austell. This has been corrected.

Steve Hoskin could not contain his excitement. He was on the phone to his old friend, Tony Williams, describing his brilliant new life in the Cornish market town of St Austell, the equivalent of a metropolis for a country boy raised in a tiny, close-knit village.

He talked about his bedsit, his first home of his own, and his beloved terrier dog. And then he revealed his exciting secret: "I'm in a gang."

"He loved the idea of being in with that lot," Mr Williams said. "He was an innocent boy who didn't have a clue what he was getting into."

Mr Hoskin, in his late 30s but childlike in many ways because of severe learning difficulties, came to consider his fellow "gang" members his best friends.

But for more than a year they exploited and cheated him, taking control of his money, his flat, his life. They treated him as their slave, made him call them sir and madam, and dragged him around his bedsit on his own dog's lead.

Then on the night of July 5 last year three of the gang, one a girl of 16, tortured him until he falsely confessed to being a paedophile. Their kangaroo court sentenced him to death, forced him to swallow 70 painkillers and frog-marched him to the top of the Trenance viaduct that soars over St Austell.

High on drugs, alcohol and power, they rolled Mr Hoskin a last cigarette before forcing him over the safety rail. As he clung on by his fingertips, the girl, Sarah Bullock, stamped on his hands and he fell 30 metres.

This week his three tormentors were jailed for a total of more than 40 years. Bullock and her boyfriend, Darren Stewart, 30, were found guilty of murder. Martin Pollard, 21, was convicted of manslaughter.

Steve Hoskin's murder has chilling echoes of the death of Kevin Davies, another gentle, vulnerable man, who was starved and beaten while being kept prisoner in a garden shed in the Forest of Dean by a couple he considered his best friends. They were jailed for 10 years each last month.

But Mr Hoskin's case may come to be seen as more significant. For as his life was taken over, both social workers and police frequently visited his flat but did not spot what was going on even though Bullock and Stewart were living there.

The Guardian has learned that a month before he was killed, contact with Cornwall's department for adult social care (DASC) stopped. "He disappeared completely from their sight," said a source involved in the case. DASC accepts contact ceased.

The source has also claimed the names of Stewart and Bullock were flagged up to both care officials and the police. Local people were concerned that drugs were being peddled from the flat. The DASC has no record of this in its case files and police say only that they visited the flat on a "number" of occasions.

Both the DASC and the police say such issues will be tackled in a case review launched by Cornwall's adult protection committee. But some critics are angry that the review will be chaired by a senior member of the DASC and say it ought to be an independent inquiry.

Some campaigners, including the charity Mencap, claim adult social care in Cornwall - and elsewhere in the UK - is failing and that there could be many more vulnerable people like Mr Hoskin barely surviving in the community.

Steve Hoskin's upbringing was as peaceful as his death was violent. He was raised by his mother, Ethel, in the village of Maudlin, near Bodmin on the edge of the Lanhydrock estate, famous for its collection of rhododendrons. His father was never on the scene. His body grew strong, but his mind less so. His reading and writing was not good and an IQ test he took a few years before his death put him in the bottom 0.4% of the UK's population. But he was cared for in Maudlin - every door in the village was open for him. His uncle and aunt lived in the house opposite.

As a young man he could not hold down regular work; instead he was given odd jobs by villagers. He helped farmers at harvest time and was paid with cider. He worked with his old friend, Morley Richards, at his animal feed business in the next village and was given corn for his chickens and coal for the fire. Mr Richards' son, Matt, recalled how he could be "led" in a benevolent way. "We would tell him what a strong chap he was and he would work twice as hard," he said. "I can imagine how he could be easily led in other ways."

Mr Hoskin's fall began in around 2004 when his mother became too ill to look after them both. The pair also fell out.

Mrs Hoskin was moved to sheltered accommodation in Launceston, 20 miles away. Digs were found for Mr Hoskin in Newquay. He frequently returned to Maudlin to see his old friends and work with Morley Richards.

In the spring of 2005, however, he was moved to the bedsit in Blowinghouse Close in St Austell, his landlords the housing association, Ocean Housing.

At first he seemed to enjoy his independence and he was not seen so often in Maudlin. As Tony Williams said, he thought he had found his "gang".

Unfortunately the gang leader, "Daz" Stewart, turned out to be a violent control freak. He had no home of his own, preferring to doss down on other people's floors. His front room was the street corner in front of the block where Mr Hoskin found himself installed.

Stewart was a magnet for youngsters, especially young women whom he would ply with drugs and alcohol. He was also obsessed with painkillers and the idea of leaping from bridges. Within weeks of Mr Hoskin moving into Blowinghouse Close, he had a lodger, Stewart, who began to control Mr Hoskin, partly by supplying him with alcohol and drugs. He systematically fleeced him of his benefit money.

If Mr Hoskin resisted he would punch and kick him. Soon Bullock also moved in. She was in thrall to Stewart and followed him as he abused Mr Hoskin. After the murder, the pair returned to the bedsit and Bullock pestered Stewart for sex - "playtime" as she called it.

Still, there appear to have been chances to save Mr Hoskin.

Carol Tozer, director of adult social care in Cornwall, said workers used to visit him weekly and - according to the case file - had no cause for concern. She said Mr Hoskin was "supported" until about a month before his death. He told them he did not want to see them any more. The review will look at whether they should have been so quick to comply with his wishes.

Dr Tozer said the department was "devastated" by Mr Hoskin's death and would learn lessons from it.

Reg Broad, chairman of the east Cornwall branch of learning disability charity, Mencap, called for a public inquiry. He asked: "How many more people like Mr Hoskin are out there right now?"

Mr Broad also pointed out the ugly coincidence that on the same day Mr Hoskin was abused and murdered, a report was published that found "institutional abuse" of 200 adults with learning disabilities living at a treatment centre and 46 houses in Cornwall.

Carers employed by the Cornwall Partnership NHS trust persistently abused a blind and deaf man with a low IQ, tying him up for 16 hours a day, the Healthcare Commission and Commission for Social Care Inspection revealed.

Ocean Housing agreed with Mr Broad that the review should be independent rather than one headed by social services officials. David Renwick, chief executive of Ocean, also criticised the time it would take - it is not due to report until December. "It's a matter of restoring public confidence," he said.

Friends say Mr Hoskin's mother is heartbroken at her son's death. His uncle, Thomas, said on behalf of the family: "Steven was tortured, beaten and eventually murdered by a group of people who took advantage of his inability to recognise evil in people.

"Steven led a simple life in many ways and our family were devastated to hear of his untimely death. We now hope he can finally rest in peace".

Darren Stewart, 30, jailed for 25 years for murder

The ringleader. Reviled by people who lived in and around Blowinghouse Close in St Austell but seen as a "big man" by the local teenagers. Used to hold court on the street corner. Many thought he dealt in drugs. He certainly took amphetamines and supplied alcohol to youngsters. A string of teenage girls appeared fascinated by him. One accused him of once threatening her with a hammer. While in prison awaiting trial he wrote to Sarah Bullock asking her to marry him. Police think he was simply trying to keep her under his control.

Sarah Bullock, now 17, detained for 10 years for murder

Became obsessed with Stewart and would do anything to impress him - including stamping on Steve Hoskin's hands to make him fall from the viaduct. The third defendant, Martin Pollard, described Bullock in court as "evil and vicious". She is thought to be the one who put the dog's lead on Mr Hoskin, telling him: "Time for walkies." As he sentenced her, Mr Justice Owen said Bullock was "totally infatuated" with Stewart but was still an "enthusiastic participant" in the torment. After moving into Mr Hoskin's flat with Stewart, she started drinking heavily and took drugs. Bullock's family, who live nearby, tried to get her away from Stewart two days before the murder but Stewart had turned her against them.

Martin Pollard, 21, jailed for eight years for manslaughter

The only one of the killers to have a job - he was a stonemason - and a stable address. He lived with his family in Bodmin and was close to his twin brother. He joined in the abuse after being convinced by the others that Mr Hoskin was a paedophile, a completely false allegation. He daubed graffiti claiming Mr Hoskin was a child abuser on the walls of his flat on the night of his death. Pollard is the only one who appears to have shown remorse, and claims he is haunted by the sights and sounds of that night. "I should have stopped it from the beginning," he said in court.